Battery jars, i.e., battery cell containers, typically house a single battery cell of a multi-cell flooded lead-acid battery. Each battery cell comprises a set of at least two lead plates immersed in an acid electrolyte. The plates are arranged in parallel relation and have alternate positive and negative charges. Generally, a pair of lead bars mechanically and electrically connect the upper ends of the positive and negative plates together respectively.
Because the electric batteries depend on chemical reactions between ions carried by the plates and the electrolyte to produce electrical power, it is normal for some by-products of these reactions, e.g., lead sulfate, to become dislodged and settle to the bottom as residue. If sufficient residue (mud) settles, a conductive ridge may buildup and short-circuit the battery.
In order to prevent shorting between the plates as material flakes off, supports (bridges) are required to raise the bottom of the plates above the floor of the battery jars. The amount of mud that can be accommodated by a bridge structure depends on the height of the bridge and on lateral passages through the bridge structure to allow the mud to spread out. Ideally, the bridge should allow the mud to spread uniformly over the entire bottom of the jar. This results in the least bridge height required to accommodate the amount of mud expected during the life of the battery. By minimizing bridge height, the electrical storage capacity for a particular size of battery cell is maximized.
It is also necessary that mud does not accumulate on top of the bridges. Consequently, the tops of the bridge members are made as narrow as practicable, taking into account the compressive strength of both the lead in the plates and the material used in the bridges. The tops of the bridge members are rounded to minimize the accumulation of mud while providing bearing area. For motive-power cells using polypropylene bridges, the radius at the crest of the bridge members is typically 0.040 inches to 0.080 inches. The sides of the bridge members are made steep so that mud will slip off them.
A bridge may either be integrally molded into a jar or molded as a separate piece and inserted into a jar. Separately-molded bridges can be made with ample lateral passages for the migration of mud. Also, separately-molded bridges, although more expensive, enable a battery manufacturer to make cells having different capacities by using bridges of different heights in one size of jar.
Some prior art molded-in bridges have been in the form of bars which run perpendicular to the width of the plates. By way of example, lead acid motive-power cells, e.g., forklift batteries, typically use bridges with either two, three or four bars supporting the plates. However, the bars form a plurality of enclosed areas which block the flow of electrolyte and, therefore, prevent mud from being able to migrate from one area to the next.
Another prior art design, which allows residue to migrate more completely, is disclosed in U.S. Pat. No. 3,338,452 (Oakley). Oakley discloses bridge members comprising a plurality of ribs that are disposed on the inner surface of the bottom portion of the battery jar. The ribs are formed integrally with the bottom. They are disposed in mutually parallel sets at an angle to the wall of the jar.
A flooded lead-acid battery cell normally sits upright at all times. Thus, the weight of each plate is supported by upward force exerted on the area of contact between the bottom edge of the plate and the crest of the bridge structure. The contact force between a plate and the crest of the bridge does not result in much deformation of the bottom edge of the plate or the crest of the bridge under static conditions. However, a battery may undergo impacts and vibratory forces on its bottom during handling and shipping. Also, in its normal service, a battery used in an automotive vehicle is subjected to vibration and impacts due to irregularities in the surfaces over which it travels. As a result, the peak contact force between the bottom edge of a plate and the crest of the bridge may be many times greater than the static force. The crests of the bridge members will indent until the bearing surfaces in contact with the bottom of the plates are large enough to withstand the impacts without further deformation.
It is desirable that the bridge support members be slender, preferably no thicker than the bottom wall of the jar. If they are thicker than the bottom of the jar, molding time for integrally molded bridges will be lengthened. Thick bridge members also reduce the space available for mud for a particular bridge height.
In motive-power lead-acid batteries, bridge heights typically range from 0.5 inches to 1.5 inches. Problematically, however, bridge members, e.g., bar shaped or a plurality of ribs as disclosed in Oakley, whose height to thickness ratio exceeds about 6 to 1 may buckle during severe impacts to the bottom of the cell. The higher bridge members need to be configured to resist columnar buckling.
There is a need, therefore, for improved battery jar bridges to provide improved bottom support to lead plates of a battery cell.